Abstract

Pumping test results are often analysed on the assumption of radial symmetry. This may be because\ud
analytical solutions are used, which are limited to two-dimensional problems, or because of the\ud
widespread acceptance that this assumption is satisfactory when estimating average values of the aquifer\ud
hydraulic characteristics. A three-dimensional R-θ-Z model, built using an object-oriented approach, is\ud
used to investigate the effects of spatial heterogeneity on time-drawdown curves. Particular attention is\ud
paid to the variation of aquifer properties in the circumferential direction, a factor ignored in most pumping\ud
test analyses.\ud
The mechanisms in the numerical model include a logarithmically increasing mesh spacing in the radial\ud
direction, features operating at the pumped borehole and a moving water table. The model is applied to\ud
two and three-dimensional idealised aquifers to establish the impact of aquifer heterogeneity. Threedimensional\ud
aquifer heterogeneity is shown to produce time-drawdown curves that can be matched using\ud
two-dimensional numerical models, giving a misleading interpretation of the aquifer flow processes. For\ud
example, a delayed response to pumping at an observation borehole due to a combination of low radial\ud
permeability and a fracture in the circumferential direction can be mistaken for large aquifer storage when\ud
radial symmetry is assumed. A field example is presented where this mechanism is believed to operate